A possible attenuated strain of avian leukosis virus (ALV) will be tested in susceptible chickens to determine its pathogenicity and immunogenicity. The effect of persistent viremia with attenuated virus on challenge with virulent ALV will be determined. Pathogenesis of oncogenic and "slow virus" manifestations of avian osteopetrosis virus will be examined. A new human papovavirus will be characterized with regard to host range and growth cycle. Its relationships to other papovaviruses with respect to antigenic properties and DNA homology will be determined. The oncogenic properties of the human papovavirus, and the relatonship of viral genome to the transformed cell will be investigated. Immunological studies will be conducted to elucidate the association of this virus with human oncogenic and non-oncogenic diseases.